Many patients with diabetes frequently have to measure their blood glucose level, or glycemia. If they detect a state of hyperglycemia they immediately have to take medication to regulate their glucose level. To simplify the daily life of these patients, numerous miniaturized glucose measuring devices which can be used by a layperson have appeared on the market.
The implantation of insulin pumps in diabetics has also been proposed. These insulin pumps have to be supplied with devices for measuring glucose that can also be implanted and which, as a function of the glycemia measured, supply information to the pump and possibly start it operating.
The majority of these devices for measuring glycemia use an enzyme specific to glucose--glucose oxidase (GOD).
As shown in the appended FIG. 1, GOD is a flavoprotein (obtained for example from moulds) which catalyses the oxidation of glucose, in this case for example blood glucose, into gluconolactone, with the formation of hydrogen peroxide H.sub.2 O.sub.2, starting from the molecular oxygen O.sub.2 present in the solution to be tested, in this case blood.
This enzyme (GOD) and oxygen have thus frequently been used in devices for measuring glucose in which the oxidation of the glucose was detected by an electrical or optical transducer.
Similarly, this enzyme (GOD) and oxygen have frequently been used in amperometric devices and their use is described in the literature.
These amperometric devices comprise on the one hand a measuring apparatus provided with at least two electrical contacts connected to an ammeter and to display means and, on the other hand, a sensor which may be disposable and which can be connected to these two electrical contacts. This sensor comprises at least two electrodes: a reference electrode and a measuring electrode. The measuring electrode comprises a metal conductor coated with an enzyme specific to the product to be detected.
The appended FIG. 2 illustrates the chemical reactions occurring on the surface of this measuring electrode. When the solution to be tested is deposited on the measuring electrode, the product to be tested (in this case glucose) reacts with the enzyme (in this case the oxidized GOD) located on the electrode to form gluconolactone while the GOD passes into the reduced state [GOD(H.sub.2).sub.(red) ]. This reduced GOD then reacts with oxygen O.sub.2 which passes into the reduced state H.sub.2 O.sub.2 and which then transfers two electrons e.sup.- towards the electrical conductor C, the potential of which is fixed and is in the region of 650 mV. The fact that it is necessary to work at elevated potentials causes additional interference problems. The oxygen thus plays the part of mediator since it permits the transfer of electrons. This transfer of electrons, which is proportional to the amount of glucose present in the solution to be tested, is then measured by the ammeter and the amount of glucose present in the solution is displayed by the display means of the measuring apparatus.
Additional research has shown that amperometric devices using non-physiological, organic, inorganic or organometallic mediators can supplant devices using oxygen as the mediator. Indeed, as shown in FIG. 2, devices using oxygen as the mediator cannot be used in solutions where the stoichiometric oxygen content is less than the concentration of the component to be measured. Otherwise, in this case, while the total amount of the component to be measured is able to react with the oxidized enzyme to form the reduced enzyme, only part of the total amount of the reduced enzyme can react with the oxygen present, in proportion to this amount of oxygen. The rest of the reduced enzyme is unable to react and the quantity of electrons transmitted to the conductor C is less than it should be.
Consequently, when this type of device is used, one is either restricted by the respective concentrations of the oxygen and the component to be measured, or compelled to use a membrane to limit the diffusion of said component. This explains why attempts have been made to produce amperometric devices using a specific mediator to replace oxygen.
Very many mediators have been proposed in the literature, such as monomeric ferrocenes (Cass, A. E. G. et al (1984), Anal. Chem. 56, 667-671; Degani, Y. and Heller, A. (1987), J. Phys. Chem. 91, 1285-1289), ferrocenes grafted onto a polymer (Foulds, N. C. and Lowe, C. R. (1988) Anal. Chem. 60, 2473-2478), charge transfer conducting salts (Albery, W. J. Bartlett, P. N. and Craston, D. H. (1985) J. Electroanal. Chem. Interfacial. Electrochem. 194, 223-235), nickel cyclamates (Taniguchi, I., Matsushita, K., Okamoto, M., Collin, J-P and Sauvage, J-P (1990) J. Electroanal. Chem. Interfacial. Electrochem. 280, 221-226) and organic components such as quinones and benzoquinones (Kulys, J. J., and Cenas, N. K. (1983) Biochim. Biophys. Acta 744, 57). Because of major work by Hill et al, for example Frew, J. E., and Hill, H. A. O. (1987) Phil. Trans. R. Soc. Lond. B316, 95-106), the family of ferrocene components has become widely established and used as mediator for GOD and other flavoproteins. As a result, a sensor currently on the market is known to use a member of the ferrocene component family as mediator.
Unfortunately, mediators currently available rarely have the requisite ideal properties, namely an electrochemical potential adapted to the selected enzyme, adequate solubility and good chemical stability to light, temperature and pH and rapid interaction with the selected enzyme.
Moreover, the oxygen that may be present in the solutions to be tested competes with some mediators according to the diagram in the appended FIG. 3. While the mediator Med present on the conductor C continues to react with some molecules of reduced GOD, it is possible that a certain amount of the oxygen O.sub.2 which may be present also reacts with other molecules of reduced GOD to form H.sub.2 O.sub.2, as previously shown in FIG. 2. When measurements are made with a small potential between the measuring electrode and the reference electrode, the H.sub.2 O.sub.2 traps the electrons derived from the reaction between the GOD and oxygen and these electrons no longer pass towards the electrode. Since the amount of oxygen in solution can vary, the amount of trapped electrons also varies. As a result, there is no longer any proportionality between the quantity of electrons passing towards the electrode and the amount of glucose in the solution to be tested. Under this conditions, these sensors consequently do not give reliable results.
It is an object of the invention to overcome the above mentioned disadvantages.